feat(block2): social settings + offers cubit factory (area, relays)

Persistence + wiring the offers screen needs, per the product decisions:
- SocialSettings (keystore-backed, no plaintext): coarse area geohash +
  configurable relay URLs, both empty by default (no bundled public relays —
  don't leak metadata to third parties). Registered in DI.
- createOffersCubit(): opens a SocialSession on the first configured relay when
  online, else a null-transport cubit that degrades gracefully. OffersCubit now
  closes the owning session on dispose.

4 settings tests green; offers/social suites still green.

apps/app_seeds/lib/services/social_settings.dart

@@ -0,0 +1,41 @@
+import '../security/secret_store.dart';
+
+/// User choices for the (Block 2) social layer: the coarse area to publish/
+/// browse offers in, and which community relays to talk to. Backed by the OS
+/// keystore (via [SecretStore]) to honour "no plaintext at rest" — no
+/// shared_preferences.
+///
+/// Both default to "unset": empty area and no relays mean the social layer is
+/// simply unavailable, and the app stays fully usable offline.
+class SocialSettings {
+  SocialSettings(this._store);
+
+  final SecretStore _store;
+
+  static const _areaKey = 'tane.social.area_geohash';
+  static const _relaysKey = 'tane.social.relays';
+
+  /// The user's coarse area as a low-precision geohash (may be empty). Set
+  /// manually or from device location; the transport coarsens it further.
+  Future<String> areaGeohash() async => (await _store.read(_areaKey)) ?? '';
+
+  Future<void> setAreaGeohash(String geohash) =>
+      _store.write(_areaKey, geohash.trim().toLowerCase());
+
+  /// Configured community relay URLs (empty by default — the user adds their
+  /// own; we bundle none, to avoid leaking metadata to third-party relays).
+  Future<List<String>> relayUrls() async {
+    final raw = await _store.read(_relaysKey);
+    if (raw == null || raw.isEmpty) return const [];
+    return raw.split('\n').where((s) => s.trim().isNotEmpty).toList();
+  }
+
+  Future<void> setRelayUrls(List<String> urls) => _store.write(
+        _relaysKey,
+        urls.map((u) => u.trim()).where((u) => u.isNotEmpty).join('\n'),
+      );
+
+  /// Whether the social layer has enough config to attempt going online.
+  Future<bool> get isConfigured async =>
+      (await relayUrls()).isNotEmpty && (await areaGeohash()).isNotEmpty;
+}